1. Field of the Invention
The present invention relates to a technology for scanning a target surface with a light emitted from a light source including a surface-emitting laser array.
2. Description of the Related Art
Conventionally, there are known optical scanning devices provided in image forming apparatuses such as copiers, facsimiles, printers, and optical plotters. The optical scanning device emits a light from a light source based on information corresponding to an image to be formed and scans a surface to be scanned on an image carrier with the emitted light. The optical scanning device is disclosed in, for example, Japanese Patent Application Laid-open No. 2008-033062, Japanese Patent Application Laid-open No. 2002-341699, and Japanese Patent Application Laid-open No.2003-260813. The optical scanning device disclosed therein includes an optical system that guides the light emitted from the light source to the surface to be scanned.
The optical system is formed with various optical devices such as a deflecting unit being a polygon mirror or the like, a cylindrical lens, and a scanning lens. However, the optical devices have variations in each optical characteristic such as light transmittance and light reflectance, and the optical characteristics vary depending on an incident angle of light on each optical device, or positions where the optical devices are arranged are difficult to be made perfectly identical to each other between optical scanning devices. From these reasons, even if the optical scanning devices having an identical configuration, there are variations in a ratio of an intensity of scanning light on the surface to be scanned to a light emission intensity of the light source, or in light-use efficiency.
Because the variation in the light-use efficiency may cause variation in the intensity of scanning light, and further cause variation in image quality, this doest not meet requirements for formation of high-quality images. Therefore, a technology for reducing the variation in the intensity of scanning light is proposed. The technology is such that the intensity of the scanning light is made uniform by giving a certain allowance to the light emission intensity of the light source and controlling the light emission intensity.
However, by using the technology, the light emission intensity is caused to decrease in any device with high light-use efficiency. If the light emission intensity is decreased, droop characteristics become worse, and light emitting properties of the light source may thereby become unstable. The unstable light emitting properties of the light source cause the light emission intensity to vary and a beam spot size to be degraded, which exerts an effect on an output image.
In contrast, there are known technologies for enabling reduction of the variation in the light-use efficiency itself. As one of the technologies is such that by reducing light transmittance of optical devices such as a cylindrical lens, the variation in the light-use efficiency is reduced (see, for example, Japanese Patent Application Laid-open No. 2008-033062). By minimizing the variation in the light-use efficiency itself, the light emission intensity of the light source does not have to be reduced to such an extent that the light emitting properties of the light source become unstable. It is therefore expected to reduce the effect exerted on the output image and also to minimize the variation in the light emission intensity.
Meanwhile, there is known a technology for actively changing a moving speed of an image carrier such as a photosensitive element and changing an image forming speed called “process speed” or the like in the image forming apparatus. Namely, there is known a technology in which when an image is to be formed on an OHP transparency or a thick paper by the image forming apparatus, the image forming speed is reduced more than a case in which an image is to be formed on a plain paper (see, for example, Japanese Patent Application Laid-open No. 2002-341699 and Japanese Patent Application Laid-open No. 2003-260813). This technology has advantages such that a bright image is obtained when an image is formed on a sheet such as the OHP transparency and such that glossiness is improved. Especially, when a color image is formed by superimposing toner images formed with different colors, such as yellow, magenta, cyan, and black, on one another, this technology is suitable for preventing color muddiness to obtain a bright image by preventing reflection on boundary between toner particles by reducing the image forming speed and sufficiently resolving toners of the colors.
As explained above, in the technology for changing the moving speed of the image carrier to change the image forming speed, it is important for prevention of a decrease in image quality to control an exposure amount of the image carrier according to the change in the moving speed. For example, if the rotational speed of the image carrier is reduced to one-half or one-third of an original speed, then it is required to reduce a light emission intensity and a light emission time of the light source, so that a light emission intensity in total is set to one-half or one-third of the original intensity. To control the exposure amount, a technology for controlling a light emission intensity of the light source in the optical scanning device is proposed (see, for example, Japanese Patent Application Laid-open No. 2002-062725), and a technology for intermittently using a polygon mirror is proposed (see, for example, Japanese Patent Application Laid-open No. 2003-260813).
However, these technologies cannot sometimes support the change in the moving speed of the image carrier. For example, in the former of these technologies, the droop characteristics may be worsen similarly to the technology, while in the latter of them, the control of the exposure amount cannot sometimes be dealt with by intermittently using the polygon mirror depending on how the moving speed of the image carrier is changed. Therefore, further development of the technologies is demanded.
Especially, when a surface-emitting laser array in which a plurality of surface-emitting laser devices are arranged is used as the light source, the problem may easily arise because of a narrow light output range.
When the surface-emitting laser array is used as the light source, because the light output range is narrow, the following problems may also arise. The problems include those in a case of correction of shading properties and a case of dealing with time degradation of the image carrier and environmental changes thereof.
The shading properties are explained below. Exposure intensities are nonuniform in each central portion and edges of the polygon mirror and optical devices in a main scanning direction due to their optical characteristics. The nonuniformity is comparatively large because it is caused by a product of the optical characteristics and the light-use efficiency. To correct the shading properties, it is necessary to change the output power of the light source so as to cancel out the comparatively high nonuniformity. However, if the light output range is narrow, then it is impossible to obtain the output power such that the nonuniformity is sufficiently canceled out.
The time degradation and environmental changes of the image carrier are explained below. The output power of the light source is determined after the device is assembled allowing for the light-use efficiency. The output power needs to be controlled according to situations when the device is actually used, allowing for the time degradation and environmental changes of the image carrier. The control is implemented by, for example, process control. However, the narrow light output range causes the control to be inadequately performed.
As a result of intense study, the inventors of the present invention have found that by controlling an exposure amount in the following manner, the control can support the change in the moving speed of the image carrier although there arises some problem as follows. The exposure amount is controlled by using a plurality of light-intensity control devices called “neutral density (ND) filters” with different light transmittances, selecting a light-intensity control device with an appropriate light transmittance according to the change in the moving speed of the image carrier, and causing the selected light-intensity control device to enter an optical path from the light source to the image carrier. It is noted that there is known a technology for selectively using the light-intensity control devices with different light transmittances in fields other than the field of optical scanning devices (see, for example, Japanese Patent Application Laid-open No. 2000-241867).
The problem is such that when a light-intensity control device is simply placed in the optical path, a reflected light by the light-intensity control device placed in the optical path may exert an effect on image formation. A technology for controlling a light emission intensity of the light source by detecting the intensity of light emitted from the light source is sometimes used in the optical scanning device. When this technology is used, the reflected light by the light-intensity control device is detected together with the light emitted from the light source, and control accuracy of the light emission intensity is thereby decreased, which causes uneven image density, to be affected on the image quality, and this is a problem. Further, if the reflected light enters the image carrier, then a so-called “ghost” image is formed, which affects the image quality, and this is also a problem. These problems may arise by the reflected light in the optical device also in the technology for reducing the light transmittance of the optical device such as the cylindrical lens.
Thus, when the light-intensity control devices with different light transmittances are selectively used, it is desirable to take these problems into consideration.
There are also known technologies allowing reduction of variations of the light-use efficiency itself, which are disclosed, for example, in Japanese Patent Application Laid-open No. 2008-033062, Japanese Patent Application Laid-open No. 2001-305460, and Japanese Patent Application Laid-open No. 2006-235213, and Japanese Patent No. 2534656. One of the technologies is a technology for reducing variation in the light-use efficiency by decreasing the light transmittance of the optical device such as the cylindrical lens (see, for example, Japanese Patent Application Laid-open No. 2008-033062). If the variation in the light-use efficiency itself is suppressed, the light emission intensity of the light source does not have to be reduced to such an extent that the light emitting properties of the light source become unstable. It is therefore expected to reduce the effect exerted on the output image and also to minimize the variation in the intensity of scanning light.
However, the technology for reducing the variation in the light-use efficiency by decreasing the light transmittance of the optical devices is designed to reduce the light transmittance of the optical devices originally provided in the optical system, in which target values of rates to reduce the light transmittance are equally set. Therefore, although a range of the variation in the light-use efficiency is decreased, the light-use efficiencies cannot be controlled according to actual values of the light-use efficiencies in the optical scanning devices, and thus, the intensity of scanning light may also be inadequately controlled.
For example, in an image forming apparatus that includes a plurality of image carriers and forms a color image as a composite image of images of the colors formed on the respective image carriers, the images of the colors need to be formed so as to achieve excellent quality of the image obtained as the composite image. To achieve the quality, the image carriers need to be scanned with respective intensity of scanning light suitable for obtaining such images of the colors as above. However, if the target values of the rates to decrease the light transmittances are equally set, then it is difficult to scan the image carriers with the respective intensity of scanning light suitable for obtaining such images of the colors as above, and therefore the quality of the image obtained as the composite image may possibly be decreased.
Meanwhile, a method of controlling the light transmittance includes a method of controlling light reflectances in a light-entrance side and in a light-exit side of the optical device. When the light reflectances in the both sides are to be controlled, each light-use efficiency is determined by a product of the respective reflectances. As a result of intense study, the inventors of the present invention have found that even if the light-use efficiencies are the same as each other, the intensity of so-called “ghost” light is changed due to a correlation between the reflectances in the light-entrance side and the light-exit side. The ghost light appears when the reflected light produced in the optical device is inclined with respect to the optical axis of an incident light, and the image carrier is thereby scanned at a position different from an original scanning position. Thus, the ghost light becomes a direct factor to cause irregularities in an image and therefore needs to be avoided as much as possible.
However, the ghost light appears not only when a light-incident plane and a light-exit plane of the optical device are curved and when these planes are intentionally inclined with respect to the optical axis of an incident light, but also when there is an error in installations of optical devices or the like. Therefore, the ghost light is difficult to be perfectly removed, and thus the intensity thereof is desirably reduced as much as possible. However, if, for example, the cylindrical lens is used to reduce the variation in the light-use efficiencies simply by reflection on the incident plane and the reflection plane, then this causes the intensity of the ghost light to be increased in addition to the problems, which is not preferred.